The interactions of disc membrane proteins in vertebrate rod outer segments are being studied. This system contains a light/ATP activated cyclic GMP phosphodiesterase which we have described and purified to homogeneity. We are now pursuing the details of light-dependent activation of this protein and the details of its interactions with other disc membrane components. We are also examining fragments of cholera toxin subunit A1 which are capable of activating adenylate cyclase.